How to use LVM snapshot to clone CentOS 5 server

Jephe Wu - http://linuxtechres.blogspot.com

Environment: 36G IDE hard disk with CentOS 5.5 default installation which has /boot as /dev/hda1 and the rest are on LVM, this is server A.
Objective: to clone to another same kind of machine by using LVM snapshot. This is server B.
Tools used: RIP(Recovery Is Possible) CD V8.9, create LVM snapshot for / partition before using tar to copy to another machine.

Concept:
1. use RIP to boot up the destination machine
2. create partitions same as the original one and make file system for all partitions
3. use external hard disk or thumb drive on original machine to extend the volume group
4. create LVM snapshot logical volume on original machine before using tar to copy the whole file system
5. use tar on destinaion machine to copy over the whole file system from original machine
6. install grub on destiniation machine. reboot


Steps:
1.  use RIP to boot up the server B, choose skip the keyboard map.
2.  setting up the environment of RIP for network use
login as root without password
passwd root
ifconfig eth0 10.0.0.2 netmask 255.255.255.0 up
/usr/sbin/sshd

3.  copy over the partition configurations from server A (10.0.0.1/24)
ssh 10.0.0.1 'sfdisk -d /dev/hda' | sfdisk [--force] /dev/hda
or
use fdisk -ul /dev/hda on server A to get the sector layout, then configure it on server B

mkfs -t ext3 /dev/hda1
mkfs -t ext3 /dev/VolGroup00/LogVol00
mkswap /dev/VolGroup00/LogVol01
e2label /boot /dev/hda1

4. create snapshot on server A for intact tar backup over ssh.
4.1 extend the current volume group first
use external hard disk or thumb drive as the extra space for snapshot, as long as the thumb drive can hold the extra changes between the time you created snapshot and the time you finish then delete snapshot, thumb drive doesn't have to be same size as hard disk.

How LVM snapshot works?
As soon as you create a snapshot, LVM creates a pool of blocks. I believe that this pool also contains a full copy of the metadata of the volume. When writes happen to the main volume, the block being overwritten is copied to this new pool and the new block is written to the main volume. This is the 'copy-on-write'. Because of this, the more data that gets changed between when a snapshot was taken and the current state of the main volume, the more space will get consumed by that snapshot pool.

4.2 create snapshot (I used a 4G thumb drive as /dev/sda1)
vgextend VolGroup00 /dev/sda1
vgdisplay -v
lvcreate -n backup -l 126 VolGroup00
mkfs -t ext3 /dev/VolGroup00/backup
mkdir /backup
mount /dev/VolGroup00/backup /backup/
vcreate -l 126 -s -n rootsnapshot /dev/VolGroup00/LogVol00

4.3 to remove snapshot later, do:
umount /backup/
lvremove /dev/VolGroup00/backup
vgreduce VolGroup00 /dev/hda1
or
pvremove /dev/sda1
vgreduce --removemissing
vgreduce --removemissing VolGroup00

Note: If the physical volume is still used you will have to migrate the data to another physical volume using pvmove.

5.  copying the whole file system to server B
on server B:
mount /dev/VolGroup00/LogVol00 /mnt/hda2
mount /dev/hda1 /mnt/hda2/boot
scp 10.0.0.1:/etc/passwd /etc/
scp 10.0.0.1:/etc/group /etc/
ssh 10.0.0.1 'cd /backup; tar cvpf - .' | tar xvpf -

6. make grub
cd /mnt/hda2
chroot .
[MAKEDEV hda]
[vgchange -a y] to activate all LVM - optional
grub-install hd0

7. References:
http://www.howtoforge.com/linux_lvm_snapshots